Intensity fluctuations of a light source can markedly degrade the sensitivity and dynamic range of an interferometric measurement system such as an optical coherence tomography (OCT) system. In frequency-domain data collection systems, intensity noise can be particularly troublesome. This occurs because the high-speed tunable lasers often used in such systems may exhibit rapid wavelength-dependent gain fluctuations. These fluctuations can occur while tuning over a wide spectral band. The conventional method for mitigating the effects of source intensity noise is to combine signals produced by a pair of photodetectors at the output of a balanced interferometer, as illustrated in FIG. 1. FIG. 1 depicts a passive system 1 in the sense that active feedback or other changes over time with respect to component properties or input signals are not monitored or used. The system of FIG. 1 can be used as the front end for an OCT data collection system.
In an ideal balanced interferometer, the phase-coherence component of the reference field interferes with the phase-coherent component of the sample field (shown on the left side of FIG. 1) to generate a pair of signals with opposite phases at the outputs of the photodetectors shown as two balanced photodiodes in FIG. 1. The two input intensities I+ and I− of light from the sample field and reference field are coupled and then directed to the photodiodes as shown. When subtracted, these two coherent signal components produce an interference signal with double the amplitude of the individual signals.
Further, the incoherent intensity fluctuations of the light source cancel, on average, after subtraction. In practice, however, complete suppression of the intensity fluctuations of the source is difficult to achieve. The difficulty arises because the photocurrents must be equalized and subtracted precisely. This subtraction process is performed in a passive manner for all input frequencies for the system of FIG. 1. In practice, the amplitude of intensity noise can be hundreds of times greater than the coherent signal amplitude. As a result, the common-mode rejection ratio (a measure of the extent signals common to the inputs of a device are rejected) of the balanced photodetectors and photoamplifier must remain high over a wide frequency band.
In the arrangement illustrated in FIG. 1, deviations from the ideal 50:50 split ratio of the fiber-optic coupler, in addition to differences in the responsiveness and coupling efficiencies of the photodiodes, typically result in photocurrent imbalances of at least a few percent. Even if these imbalances can be compensated by manual adjustment at the time of system manufacture, imbalances reappear over time as a result of long-term drifting in the properties of the components. Consequently, passive balancing methods are not adequate for use in many applications.
Accordingly, a need therefore exists for noise reduction methods, apparatus, and systems that overcome these limitations.